1. Field of the Invention
The present invention relates to systems for measuring moisture and, more specifically, to a capacitive moisture sensor that provides a highly linear output throughout a large range of moisture levels, and to a system for controlling multiple moisture sensors from a central location.
2. Description of the Background
Moisture measuring systems are typically used for monitoring irrigated land and for controlling irrigation equipment. Such systems have long used a capacitive sensor comprising two parallel plate electrodes that are embedded in the soil, which functions as a dielectric. The capacitance defined by the electrodes and intervening soil varies with the dielectric constant of the soil, which in turn varies with the moisture level of the soil. The system can thus measure the capacitance and convert the measured value into a measure of moisture level in the soil. However, the presence of salts and other impurities in the soil can substantially impair measurement accuracy in such a system because impurities alter the dielectric constant; the presence of salts will produce the same effective capacitance as a larger volume of water.
U.S. Pat. No. 4,850,386, issued on Jul. 25, 1989 to the inventor of the present invention and incorporated herein by reference, discloses a sensor that minimizes the effects of impurities. Plate electrodes are disposed in a side-by-side, i.e., coplanar, arrangement. One or both electrodes are covered with a dielectric casing or coating. When the sensor is embedded in soil, traces or paths of water will form between points on the dielectric casing above each electrode. Each path forms a point capacitor between the impinging moisture drop and the conductive electrode surface. The total capacitance of the sensor equals the sum of the point capacitances over the surface area of the electrodes because the point capacitors are effectively in parallel. Due to the thickness of the dielectric casing and the very small surface area of the droplet, the resulting capacitor has a much greater impedance than the soil along the same path, thus swamping the effect of impurities in the soil. As the number of such paths increases, the impedance between the electrodes changes as a function of moisture level in a relatively consistent and definite manner regardless of the presence or amount of soil impurities. In fact, the function relating sensor impedance to moisture level is not only definite, but it is also essentially linear over a range of moisture levels.
The range of moisture levels over which moisture level is directly proportional to sensor impedance is quite limited, however. As moisture levels increase, the electrodes become increasingly saturated with moisture traces, and the relationship between moisture level and sensor impedance becomes increasingly non-linear, saturating the sensor at a very low moisture level. This non-linearity produces inaccurate moisture level measurements. Additionally, each moisture trace has a resistance in addition to a capacitance; the measuring circuit thus oscillates in response to a complex impedance. The ratio of the resistance portion of the impedance to the capacitance portion of the impedance increases with increasing moisture levels. A sensor that exhibits a highly linear relationship between impedance and moisture level over a wide range of moisture levels would thus be desirable.
Moisture measuring systems may also control irrigation equipment. The above-cited U.S. Patent discloses a module having a moisture sensor circuit and a circuit that opens a valve when the measured moisture level drops below a predetermined level and closes it when the moisture level has returned to the same predetermined level.
Irrigated tracts such as lawns and golf courses may have many regions, each irrigated by a sprinkler or group of sprinklers. The irrigation of each region is controlled by a separate module. Practitioners in the art have improved the control systems of such modules by using microprocessors or microcontrollers executing algorithms that calculate optimal watering intervals. Although such systems are flexible and can easily be re-programmed to meet the individual requirements of each region, frequent re-programming of numerous modules located over a large irrigated area is required and must be changed as weather conditions vary.
Practitioners in the art have controlled irrigation valves from a centralized location or hub by modulating the voltage on the power line between the hub and the remote modules. Controlling the valves in this manner is advantageous because existing irrigation equipment installed in many golf courses and other irrigated tracts is connected to a 24 volt AC power line, which is a widely used standard. This modulation enables the centralized controller to remotely open and close the valves in response to a timer or a manual control switch, but the centralized controller does not receive any moisture level information.
A moisture monitoring and control system that has modules that transmit moisture level measurements to a centralized controller, which in turn remotely controls individual irrigation valves in response to these measurements, would be highly desirable.
There are also applications for moisture monitoring devices in moisture-containing media other than soils. For instance, the curing of concrete is a function of the change in moisture content of the concrete mixture. Similarly, the potential for failure of earthen dams is a function of the amount of water seepage within the dam structure. The presence and/or movement of toxic liquids in soil can be determined by measurement of water displacement by such liquids. All of these applications, and others of similar nature, some of which are mentioned below, could benefit from monitoring by use of moisture detection devices.
These problems, needs and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.